Since the invention of the transistor in 1947, much effort has been directed towards extension of the device operating range towards higher and higher frequencies.
Conventionally, the cut-off frequency f.sub.T (defined as the frequency at which the current gain .beta., i.e., the absolute value of the parameter h.sub.fe .ident..differential.J.sub.C /.differential.J.sub.B, is unity) is used as a figure of merit that is indicative of the high frequency capability of a transistor. See for instance, S. M. Sze, "Physics of Semiconductor Devices", 2nd Edition, John Wiley & Sons, 1981, Chapter 3, incorporated herein by reference. It is well known that .beta. at high frequencies decreases at a rate of 10 dB/decade, i.e. proportionally to inverse frequency.
Another parameter that can be used to characterize the high frequency capabilities of a (typically microwave) transistor is the unilateral (power) gain U. See S. M. Sze, op. cit., pp. 160-165. It is well known that U at high frequencies decreases at the rate of 20 dB/decade, i.e., proportionally to inverse square of the frequency. The frequency at which the unilateral gain is unity is the maximum oscillating frequency f.sub.max, which can, but need not, be larger than f.sub.T. Both f.sub.T and f.sub.max are conventionally determined by extrapolation of the measured roll-off in h.sub.fe and U, respectively. Although HBTs having f.sub.T substantially above 100 GHz have recently been reported (see, for instance, Y. K. Chen, et al. IEEE Electron Dev. Lett., Vol. 10, No. 6, p. 267, 1989), it would clearly be highly desirable to have available transistors that can, inter alia, operate at even higher frequencies.
G. T. Wright, (see, for instance, Solid State Electronics, Vol. 22, p. 399, 1979) proposed extension of active transistor operation to frequencies beyond the conventional cutoff frequencies. The proposal involved the utilization of transit time resonances that arise from carrier drift in the collector space charge region, resulting in a negative output resistance of the transistor. The proposed model suggested for an ideal transistor (i.e., a transistor without any parasitic extrinsic impedances) the possibility that .vertline.U.vertline. could exceed unity at frequencies above f.sub.max. However, it has now been shown (S. Tiwari, IEEE Electron Device Letter, Vol. 10, No. 12, p. 574, 1989) that the proposed utilization of the collector transit time resonances in a conventional GaAs/AlGaAs HBT would require reductions of each of the base and collector resistances and of the collector capacitance by at least an order of magnitude from state of the art values. Clearly, the proposed mechanism is, at least for the foreseeable future, not likely to be embodied in a practical device. Recent analysis shows that the indicated difficulty in utilization of the collector transit-time effect arises because of a relatively large decrease (by at least a factor of three) in the magnitude of the common-base current gain, which is in principle unavoidable if a necessary transit angle of order 180 degrees is acquired in carrier transit across the collector space-charge region. The resultant gain is so weak that it is practically damped by parasitic extrinsic impedances.
Almost 40 years ago it was suggested (W. Shockley, Bell System Technical Journal, Vol. 33, p. 799) that active transistor behavior above the conventional transit time cutoff could be obtained from the base transport of minority carriers. A necessary condition for this is that the directed minority carrier transport across the base is much faster than the diffusive transport. In principle, this condition could be met in a transistor with exponentially graded base doping profile. To the best of my knowledge, no such device has ever been realized.
U.S. patent application Ser. No. 07/981,588, filed Nov. 25, 1992 and incorporated herein by reference, discloses a "coherent" ballistic transistor capable of providing gain at frequencies above the conventional cut-off frequency. The coherent transistor employs the base transit angle and therefore is much less susceptible to the parasitic damping than previous proposals utilizing the collector transit angle. However, because of the requirement that the minority carrier transport across the base be ballistic, practical realizations of the disclosed are likely to be restricted to low-temperatures and ultra-high frequencies (exemplarily well above 100 GHz).
It would clearly be desirable to have available a transistor that can operate at room temperature, exhibiting transit time resonances at frequencies above the conventional cutoff frequencies. Moreover, it would be desirable to be able to choose the resonant frequency in a wide range, not necessarily above 100 GHz.
This application discloses such a transistor. The novel device, to be referred to as the "enhanced diffusion" transistor (EDT), has utility in many fields, e.g., high speed computation or communications.